A wide range of products, from consumer electronics to battery-powered electric vehicles, utilize electrochemical energy sources. Similarly, there is a wide range of primary and secondary alkaline batteries that have been proposed and/or used for these varying applications. As representative examples, there can be listed the following electrochemical systems: AgO/Zn, Ag.sub.2 O/Zn, HgO/Zn, HgO/Cd, Ni/Zn, Ni/Cd and Zn/air systems.
It is known that the battery separator for such electrochemical systems must possess a variety of characteristics. A principal requirement is that the electrolyte resistance characteristics should be uniform. Indeed, ideally, any particular lot of the separator material should exhibit zero variation in electrolytic resistance characteristics.
When the distribution of the electrolytic resistance is non-uniform within a given lot or quantity of the separator film (e.g., roll), cells and batteries utilizing separators made from such non-uniform lot will have variable and unpredictable electrical performance characteristics. More particularly, such non-uniform separators will result in variable current and voltage performance from battery to battery. Such non-uniform and variable performance creates substantial problems for the battery manufacturer and for the user of the battery as well. For example, a separator having a higher-than-specific resistance area will give a lower closed circuit voltage and a lower capacity than the specification for the cell or battery. Using such a separator might also result in cells or batteries that cannot meet the low temperature and high rate performance specifications set for such cells or batteries.
Considerable effort has been directed over many years to developing materials that satisfy the stringent and varied requirements for separators for electrochemical systems such as for those previously identified. In addition to the desired electrolytic resistance characteristics, the separator material must allow satisfactory cycle life and provide adequate shelf life. The separator material must have satisfactory resistance to chemical oxidation, and, depending upon the electrochemical system involved, appropriate retardation of silver and mercury ion diffusion, and adequate retardation of zinc dendrite growth.
Over the course of at least the last 20 years or so, one type of separators that have been utilized for alkaline batteries include polyethylene, polypropylene and polytetrafluoroethylene-based electrolytically conductive films. The base films, particularly polyethylene, have excellent oxidation resistance and superior chemical stability in alkali. Appropriate electrolytic resistance and hydrophilic properties have been achieved by modification of the base film using gamma radiation grafting techniques. Separators of this type have also been used for many years which are radiation crosslinked to further alter the characteristics of the base film.
There has also been substantial attention over this same 20-year time period that has been directed to examining the manner in which such radiation-grafted, electrolytically conductive polymeric films have been prepared. Seemingly, every aspect of making this type of film has been examined, as well as the effect on the performance of such films as battery separators.
Prior researchers have thus stated that the order of performing the two operations of grafting and crosslinking is important. Although it is easier to graft first and then to crosslink, test results, it is concluded, have indicated that a preferred film is made by crosslinking first, then grafting.
It has been stated that the molecular properties of the base resin or film which are important in making the films include the crystallinity, the molecular weight distribution and the absence of low molecular weight fractions. Low density polyethylenes have been preferred for many applications. It has also been shown that, with such low density polyethylenes, the cycle life of the grafted film increases as the crosslinking dose is increased. It has likewise been proposed that the cycle life at a high crosslinking dose appears to be related to the properties of the base resin.
It has been further noted that highly crosslinked grafted films swell much less than the identically grafted films which are not crosslinked and that highly cross-linked films are more difficult to graft. It has also been stated that the use of methacrylic acid as a graft monomer gives better cycle life than the use of an acrylic acid graft.
At one time, it was thought that polymeric films with low graft levels would have higher resistance but would have a greater cycle life than high graft level films, due to the decreased permeability to ions such as zinc and the like in the low graft films. Some test data, however, has caused some prior researchers to conclude that high graft films prepared from radiation crosslinked polyethylene are superior to other graft copolymer films.
Prior researchers have also opined that, during the grafting process, homopolymerization of the monomeric material used for grafting can take place and that such homopolymerization is undesirable, both for processing as well as product reasons (i.e., a lack of uniform grafting). One prior solution suggests introducing air into the grafting solution to tie up the free radicals formed during irradiation, thereby inhibiting the homopolymerization process. When a non-crosslinked base film is used with a methylene chloride solvent system for the monomeric grafting material, it has been suggested to include a chemical inhibitor, in addition to air, into the grafting solution.
Another prior solution to the homopolymerization problem suggests, when preparing a separator film from a polyethylene film using acrylic acid in water as the grafting monomer, the addition of a ferrous or cupric salt in an amount to inhibit the formation of a homopolymer of acrylic acid in the solution surrounding the polyethylene film to thereby help achieve a uniform graft reaction in the polyethylene film.
Despite all of this considerable effort in analyzing and testing this type of electrically conductive polymeric films over at least the last 20 years, the uniformity of the electrolytic resistance properties in a lot of such separator material is substantially less than is desired. Indeed, as previously noted, the problems caused by this lack of sufficiently uniform electrolytic resistance characteristics are substantial. Cells using non-uniform separators will have greatly varied closed circuit voltages which will affect the rate capabilities and yield non-uniform energy capacities.
There accordingly exists the need for both a more efficient method for making battery separators from electrolytically conductive polyethylene films as well as lots of such films that are characterized by significantly more uniform electrolytic resistance characteristics within the lot and from lot-to-lot. More particularly, to avoid the performance problems that may result from cell-to-cell when a non-uniform lot of polyethylene separator film is used, there exists a need for separator film lots that can be made having the general range of electrolytic resistance desired for the specific applications, yet which possess highly uniform electrolytic resistance values within the range desired.
As one example, many applications require polyethylene separator films having extremely low electrolytic resistance values (when measured in 40% KOH at 1000 Hz at 23.degree. C.), desirably within the range of 100 to 250 m.OMEGA.-cm.sup.2 (an average of 160 m.OMEGA.-cm.sup.2). Prior art methods cannot even consistently achieve lots of such separator films having electrolytic resistance values within the specified 100 to 250 m.OMEGA.-cm.sup.2 range, much less the desired highly uniform resistance values within the specified range.
It is therefore an object of the present invention to provide electrolytically conductive polyethylene films for use as battery separators which are characterized by highly uniform electrolytic resistance characteristics. It is also an object of this invention to provide a more facile method for making such films.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.